These creepy sharks glow bright green in the dark. Here’s why.

The largest disco on Earth is happening deep beneath the surface of the ocean, and no, you’re not invited.

New footage from National Geographic researcher David Gruber and his team reveals a shark glowing bright green 120 feet deep in Scripps Canyon off the coast of Southern California. To those of us who are used to our sharks having skin that doesn’t glow, this might come as a shock. But Gruber’s observation, though relatively new for us, is how it’s always been for these sea creatures — we just couldn’t see it until now.

The fish in question, known as a swell shark, glows thanks to a process known as biofluorescence. This essentially means that the shark is absorbing blue light and re-emitting green light. Scientists suspect that this kind of behavior has something to do with how these sea creatures attract mates.

This process is somewhat different from bioluminescence.

Bioluminescent organisms glow because of internal chemical reactions that allow them to produce their own light. (Think of the glowing waves in California or even the movie Avatar as good examples of bioluminescence in action.)

Gruber told National Geographic to think of biofluorescence as needing a black light, rather than acting like a glow stick.

Because the ocean overwhelms the human eye with darkness, past a certain depth we’re unable to see biofluorescence in action with the naked eye.

So, Gruber and his team developed a tool that actually acts as a shark’s eye, allowing them to see what a fish might observe underwater.

Researchers have used the shark eye lens to find more than 200 biofluorescent species, including some species of eels and even sea turtles.

“Imagine being at a disco party with only blue lighting, so everything looks blue,” Gruber told National Geographic at the time of the discovery. “Suddenly, someone jumps onto the dance floor with an outfit covered in patterned fluorescent paint that converts blue light into green. They would stand out like a sore thumb.”

But you don’t have to go deep-diving with National Geographic to see this in action, some forms of biofluorescence are actually visible to the naked eye.

“An example of seeing biofluorescence (without special equipment) is when diving on a deep coral reef and witnessing a red coral or fish,” Gruber said via email.

“Since ‘red’ light from the sun only penetrates a few meters deep into the ocean, if we see red at depth, it is actually the animal converting blue light into red light.”